Investigator's writing on IMP packages

Nowadays, IMP packages merely have a "medication" number which is linked automatically by an IWRS to a certain "patient" number. In some trials, patients are receiving a lot of different packages of IMP.
In order not to get lost, many investigators tend to write (wet ink) the actual patient number already on the IMP package prior to dispensing it to the patient. Often they also indicate the visit number or date, and eventually the regimen.

There seems to be a lot of discussion on the interpretation of the GMP/GCP regulations as to whether or not the investigator is allowed to do so, as he might becoming a "manufacturer" by altering the IMP drug label and thus becoming legally responsible for the IMP he/she dispensed.

In some companies, this is not being considered an issue, in others it is strictly prohibited.

Can you please share your experience with me on this topic?
If prohibited by law/regulations, where exactly can the text indicating so be found (Annex 13 doesn't state anything on this matter, nor does GCP text seem to)?
What is standard practice in other companies?
What is MHRA's point of view?